The invention relates to a method for operating a fuel cell system in a vehicle, and in particular to a method of operating a fuel cell system in which the fuel cell system is switchable into and out of a temporary stop mode.
Stop/start systems in vehicles are known from the general prior art, and are used for saving energy and reducing noise emissions by switching off the drive unit during short phases, in which no power is required, for example at a junction, a red traffic light or when the vehicle is rolling, in order to thus save energy. Typically, besides the drive unit itself, auxiliary consumers such as for example, pumps, compressors and the like are also switched off or at least brought into a state of reduced power. It is thus possible to clearly reduce both the energy requirement and the emission of harmful substances and noise.
Start/stop systems are also known in vehicles equipped with a fuel cell system for generating the necessary drive power. U.S. Pat. No. 6,484,075 B2 describes a method for placing a fuel cell temporarily in a stop mode for the duration of a vehicle stop. On the basis of a request of the vehicle to switch to a stop mode of the fuel cell system, first it is checked whether the fuel cell system is in a position to fulfill this request. Insofar as this is the case there is a switch to a stop mode, in which both the combustion gas supply and the air supply of the fuel cell are completely switched off. Energy and emissions are thereby saved. However, the method described therein has the significant disadvantage that through the constant switching on and off comparatively rapid ageing of the fuel cell arises and for a restart both of the hydrogen supply and the air supply from a complete standstill a comparatively long time is required, in which upon restart of the vehicle the fuel cell cannot supply any drive power.
Exemplary embodiments of the present invention are directed to methods for operating a fuel cell system in a vehicle that provides a more comfortable functionality during stop/start operation and simultaneously facilitates a long lifespan of the fuel cell.
Due to the fact that in the inventive method the electric contacting of the fuel cell is maintained with the switch to the stop mode it is furthermore possible for a current to be drawn from the fuel cell so that residual media, and here in particular residual oxygen, are removed in the region of the fuel cell. This avoids harmful voltages for the individual cells of the fuel cell during the stop mode, which would have a disadvantageous effect upon the lifespan of the fuel cell. In order to have a sufficient quantity of combustion gas, in particular hydrogen, available for the removal, the supply of the combustion gas to the anode side of the fuel cell is not interrupted in the inventive method but instead merely decreased to a reduced pressure. This prevents an excessively large differential pressure between the anode region and the cathode region with switched off or minimized air supply of the cathode region of the fuel cell. If the fuel cell—as is the case according to a particularly preferred further development—is formed as a PEM fuel cell this reduced pressure difference ensures that the membranes of the fuel cell are correspondingly spared and not loaded with too high a differential pressure.
According to a very favorable and advantageous further development of the inventive method combustion gas that is not consumed in the fuel cell system is fed from a region after the anode via a recirculation conveying device together with fresh combustion gas from the valve device to the combustion gas supply of the anode, wherein the switch to the stop mode further comprises the mass flow conveyed by the recirculation conveying device being reduced to a predefined value. During the stop mode, therefore, upon use in a fuel cell system with anode recirculation the recirculation conveying device continues to be operated. This can be formed according to a very advantageous further development as a recirculation fan driven by an electric motor. In this case the recirculation fan can be easily and efficiently brought to a predefined speed which is clearly lower than the speed in regular operation. Together with the negligibly maintained pressure and thus the negligible supply of hydrogen according to the inventive method such a structure is produced, wherein through the recirculation conveying device the gases continue to be moved in the region of the anode and the recirculation line so that oxygen possibly penetrating into the cathode region of the fuel cell can be correspondingly removed by loading the fuel cell with a current.
According to a very advantageous embodiment of the inventive method the fuel cell is electrically coupled to an electronic unit for removing the power of the fuel cell, through which excess power of the fuel cell is also stored in an energy storage device, whereby through the electronic unit after the switch to the stop mode an electric current is drawn from the fuel cell until a predefined voltage is reached and after which the fuel cell is kept regulated to a constant voltage. This very advantageous embodiment of the inventive method provides for the very frequent use of the fuel cell together with an energy storage device, such as a battery. In situations in which excess power of the fuel cell is available or in situations in which the vehicle is decelerated via the electric drive motor in generation operation, electric power that is produced in the system and not directly required can be intermediately stored. The structure now has the significant advantage that with the entry into the stop mode a current is removed through the electronic unit from the fuel cell. This current ensures that the voltage is kept correspondingly low and that therefore no corrosion arises in the region of the individual cells in the fuel cell. A favorable threshold for the voltage is approximately 0.85 Volt for each individual cell. By means of the current, residual oxygen situated in the cathode region is also removed as this can react with the combustion gas because the combustion gas supply according to the invention still continues at a low level. After the residual gases have been used up the voltage is held via the electronic unit at a predefined low level in order to also continue to maintain readiness for operation. In addition this low voltage level which typically then lies on the whole high voltage bus of the fuel cell system allows a reliable operation thereof as it is also responsible for example for the drive of the recirculation conveying device. It can thereby arise by all means that a negative current flows into the fuel cell, the power necessary for this can be removed from the energy storage device and is typically very low.
In a favorable further development of the inventive method the air conveying device is completely stopped upon switching to the stop mode. This structure can be realized very simply using volumetric compressors and the like, as these can restart very quickly and provide a maximum air flow, which is very favorable with regard to the noise emissions and the energy requirement.
Due to the oxygen supply of the cathode region that then no longer occurs, the voltages of the individual cells can further reduce and diverge. In order to also securely and reliably avoid in such cases a corrosion of the individual cells of the fuel cell it can be provided according to a very advantageous further development of the inventive method after the stop mode has lasted a predefined time span a temporary increase of the air mass flow is carried out, whereby after reaching a predefined value of the air mass flow there is a switch back to the stop mode. This short term running of the air conveying device temporarily produces an air mass flow into the cathode region of the fuel cell. With this so-called refresh there is then once again an increase in the voltage in the region of the fuel cell and thus a reduction in the risk of corrosion, as the mechanisms described at the start are used again. As merely the air mass flow is temporarily increased and the other processes and components remain in their state for the stop mode this refresh has a comparatively low energy requirement and only low noise emissions. However, it can also facilitate a considerable advantage having regard to corrosion and thus the lifespan of the fuel cell.
In a further very advantageous embodiment of the inventive method a flow compressor can be used as an air conveying device that continues to run during the stop mode at a predefined low speed. Flow compressors conveying the air mass flow for the fuel cell via a fan or similar are known from the general prior art in fuel cell systems and have corresponding advantages in relation to volumetric compressors. However, flow compressors require a comparatively high speed in order to be able to provide the required air mass flow. In a stop mode such a flow compressor is therefore not completely stopped but instead runs according to the invention at a low speed. This has the advantage that upon restart of the system it does not have to be so greatly accelerated, whereby this has considerable advantages in relation to the energy requirement and the time until restart of the fuel cell system. A further aspect of allowing the flow compressor to continue to run as an air conveying device at a low speed is that a low air mass flow continues to be conveyed to the fuel cell so that the abovementioned refresh can typically be omitted in this embodiment of the inventive method.
In a very favorable and advantageous further development of the inventive method it is now also provided that when switching to the stop mode between the air mass flow to the fuel cell after the air conveying device and an exhaust gas flow from the fuel cell system a flow connection is opened. Such a flow connection, which could also be described as a system bypass valve, thus creates a flow connection from a region after the air conveying device into the exhaust gas region. Without blocking off the cathode region of the fuel cell, which would necessitate very high-resource and expensive valve devices that are additionally prone to breakdown, very heavy and require a considerable construction space, it is thus possible with a single very simple valve to ensure that, for the possibly still conveyed air mass flow or the air mass flow built up through a build-up pressure for example during rolling down a hill of a vehicle, a path with a very low pressure loss is made available, via which a large part of the air mass flow can flow directly again away from the fuel cell system without previously passing the fuel cell. The quantity of oxygen conveyed into the fuel cell or its cathode chamber is thereby correspondingly reduced. It is nonetheless sufficient to prevent the abovementioned problems in relation to corrosion even without a refresh insofar as the air conveying device is formed as a flow compressor and continues to run at a low speed.